Potentiometers



Dec. 12, 1967 w. D. KIRKENDALL POTENTIOMETERS Filed Sept. 26, 1966 2 Sheets-Sheet 1 INVENTOR. W/LL/AM 0' MAKENO/ILL BY ATTORNEYS I Dec. 12', 1967 w. D. KIRKENDALL 3,358,258

POTENTIOMETERS Filed Sept. 26, 1966 2 Sheets-Sheet 2 United States 3,358,258 PQTENTHGMETERS Wiiiiarn D. Kirlrendall, Dalton, Pa, assignor to Weston instruments, Ina, Newark, Null, a corporation oi Delaware Filed Sept. 26, 1966, Ser. No. 581,942 11 Qlairns. (Ci. 333l83) This invention relates to potentiometers and, more particularly, to miniature potentiometers of the type including an elongated resistance element, a movable contact slidably engaging the resistance element, and a lead screw parallel to the resistance element and arranged to drive the movable contact rectilinearly along the resistance element.

Such potentiometers are commonly produced in very small sizes, typically employing a casing in the form of an elongated rectangle on the order of 1.25" x 0.3 X 0.2". Because of the small size of such devices, prior-art workers have found it difficult to devise a structure which can be manufactured at a reasonable cost, yet will have the desired strength coupled with the necessary precise ness and dependability of operation. Particular dilficulties have been encountered in providing a truly satisfactory arrangement for the conductive leads relative to the casing and the elements of the potentiometer housed therein, and especially in achieving adequate strength in the lead structure when the intended use is such that the leads must extend away from one side of the casing.

A general object of the invention is to provide an improved rectilinearly operating potentiometer which is specially suited for manufacture in very small sizes.

Another object is to devise an improved casing structure and lead arrangement for such potentiometers.

A further object is to provide a simple, effective, and easily assembled potentiometer structure of the type comprising an elongated resistance element, a return conductor, a movable contact unit having blades slidably engaged respectively with the resistance element and return conductor, and a lead screw for driving the contact unit.

Yet another object is to devise a casing and lead arrangement for miniature rectilinearly operating potentiometers which allows flush side-by-side mounting of a plurality of the potentiometers, yet provides increased strength in the leads.

A still further object is to provide such a potentiometer characterized by improved strength and ability to withstand shock and vibration.

Stated broadly, potentiometers according to the invention employ an elongated casing defining a cavity within which the resistance element, a return conductor, the lead screw, and the movable contact assembly are disposed, the casing having two spaced parallel opposite sides and at least a third side, the potentiometer further comprising conductive lead means, one of the parallel opposite sides having an aperture opening into the cavity at a point adjacent one end or: the resistance element, appropriate outer surface portions of the casing having a groove communicating with the aperture and extending transversely around the casing to a point within the confines of the third side, the lead means including a conductor having one end portion projecting through the aperture and electrically connected and secured mechanically, as by soldering, brazing, or welding, to the adjacent end of the resistance element, the body of the conductor extending along the outer groove, and the other end portion of the conductor projecting away from the third side of the casing. The conductor is rigidly secured in the groove, as by filling the groove with a suitable cement, and the fact that the groove embraces a considerable proportion Patent the lead 3,358,253 Patented Dec. 12, 1967 of the length of the conductor provides a markedly more stable lead structure than has heretofore been achieved. The casing advantageously consists of two halves, each molded from electrical insulating material and each being of L-shaped transverse cross-section, the resistance element being carried by one half and the return conductor by the other half, abutting edges of the two halves being secured together, as by cementing. The one of the halves which carries the return conductor includes integral longitudinally spaced end portions which close the ends of the casing and support the lead screw. The potentiometer is thus easily assembled, with the resistance element and its conductive leads being installed on one casing half, screw and the return conductor with its lead being installed on the other casing half, and the two resulting sub-assemblies then being secured together.

In order that the manner in which the foregoing and other objects are attained in accordance with the invention can be understood in detail, one particularly advantageous embodiment thereof will be described with reference to the accompanying drawings, which form a part of this specification, and wherein:

FIG. 1 is a perspective view of a potentiometer constructed in accordance with the invention;

FIG. 2 is an enlarged transverse sectional view taken on line 22, FIG. 1;

FIG. 3 is an end elevational view of a combined nut member and contact unit employed in the potentiometer of FIG. 1;

FIG. 4 is a side elevational view of the unit of FIG. 3;

PEG. 5 is a View, partly in transverse cross-section and partly in end elevation, showing the unit of FIG. 3 operatively engaged with the lead screw of the potentiometer of FIG. 1, with spring contact arms of the unit in relaxed positions;

FIG. 6 is a longitudinal sectional view taken on line 66, FIG. 2;

FIG. 7 is a longitudinal sectional view taken on line 77, FIG. 2;

FIG. 8 is a bottom FIG. 1;

FIG. 9 is a fragmentary transverse sectional view taken on line 99, FIG. 7; and

FIG. 10 is a transverse sectional view taken on line 10-10, FIG. 7.

Turning now to the drawings in detail, the casing 1 comprises half sections indicated generally at 2 and 3 respectively. Half sections 2 and 3 are each in the nature of an integral unit molded of suitable synthetic resin material, such for example as a conventional glass filled diallyl phthalate composition, having good electrical insulating properties. When assembled, casing 1 includes two-spaced opposite sides 4 and 5, a third side 6, and a fourth side 7, all four sides having rectangular outer surfaces. As seen in FIGS. 1 and 2, sides 6 and 7 are narrower than are sides 3 and 4, so that the transverse crosssection of the casing is a rectangle which is elongated in the direction in Which sides 6 and '7 are spaced apart.

Half section 2 comprises sides 4 and 6, while half section 3 comprises sides 5 and 7 plus two end portions 8 and 9. As will be clear from FIGS. 1, 2, 6 and 7, the end portions of side 4 extend across flat side faces 10 and 11, respectively, of casing portions 8 and 9 and are provided with circular openings which accommodate cylindrical projections 12 and 13 formed on end portions 8 and 9, respectively. Eores 14 and 15 are provided in end portions 8 and 9, respectively, and extend transversely of the casing, each bore l4, 15 having its axis centered relative to the corresponding projection 12, 13. The latter are of such length that, with the casing fully assembled, the ends of projections 12 and 13 lie in the plane of the outer plan view or" the potentiometer of surface of side 4. Bores 14 and 15 are provided to accommodate suitable mounting rods or bolts (not shown) when two or more of the potentiometers are to be mounted side-by-side with side 4 of one potentiometer in engagement with side 5 of the next potentiometer.

While the outer surface of side 4 is completely flat, the inner surface is of stepped configuration arising from the fact that the edge portion 16, FIG. 2, of side 4 most distant from side 6 is relatively thin, the intermediate portion 17 is thicker, and the portion 18 of side 4 adjacent side 6 is still thicker. The inner surfaces of portions 16 and 17 are joined by a shoulder 19, and the inner surfaces of portions 17 and 18 by a shoulder 20, shoulders 19 and 20 facing away from side 6. As best seen in FIG. 6, the end portions of side 4 which overlie faces and 11 of casing end portions 8 and 9 are of the same thickness as portion 16 throughout their entire extent. Accordingly, thicker portions 17 and 18 extend longitudinally only for the distance between the respective inner faces 21 and 22 of casing end portions 8 and 9.

Side 6 extends longitudinally only between faces 21 and 22 of casing end portions 8 and 9. For most of its width, this side is of the same thickness as portion 18 of side 4. The edge portion 23, FIG. 2, of side '6 most distant from side 4 is of reduced thickness, the inner surfaces of the two portions being joined by a longitudinal shoulder 24.

At points generally adjacent casing end portions 8 and 9, respectively, when the casing is assembled, side 4 is provided with circular apertures 25 and 26. A straight groove 27, of generally U-shaped transverse cross-section, is provided in the outer surface of side 4, one end of the groove opening into aperture 25, the groove extending from that aperture to the junction between the outer surfaces of sides 4 and 6 and there joining a like straight groove 28 in side 6. Groove 28 extends just beyond the longitudinal center line of side 6, terminating in a semicircular end 29, FIG. 8, the center of curvature of end 29 lying on the longitudinal center line of side 6. The central axes of aperture 25, groove 27 and groove 28 lie in a common plane transverse to casing 1. A straight groove 30 in the outer surface of side 4 communicates with the second aperture 26 and joins straight groove 31 in the outer surface of side 6, groove 31 terminating in semi-circular end 32, the combination of aperture 26, groove 30' and groove 31 being in all material respects identical to that just described for aperture 25, groove 27 and groove 28.

A conventionally formed wire wound resistance element 32, FIGS. 2 and 6, is fixedly secured to half section 2, as by being cemented thereto with epoxy cement. Element 32 is elongated and of a diameter which, as seen in FIG. 2, is markedly smaller than the space between the inner surface of portion 18 of side 4 and the inner surface of side 5. The resistance element extends longitudinally along the junction between walls 4 and 6 and, as seen in FIG. 6, is longer than the space between apertures 25 and 26 but shorter than the space between faces 21 and 22 of casing end portions 8, 9. Terminal leads for the resistance element are provided in the form of conductors 33 and 34 which are, for example, of round nickel wire plated with gold.

As best seen in FIG. 2, conductor 33 includes a straight end portion 35 which projects inwardly through aperture 25 and terminates adjacent the corresponding end portion of resistance element 32. End portion 35 is connected electrically, and secured mechanically, to the resistance element by solder at 36. The outer end of portion 35 joins a straight portion 37 of the conductor in a right angle bend 38. Straight portion 37 extends through groove 27 and joins a second straight portion 39 in a right angle bend 40, portion 39 extending through groove 28 and joining an elongated straight end portion 41 in a right angle bend 42 such that portion 41 projects away from side 6 and lies in a plane whichis at right angles to that side and contains the longitudinal center line thereof. Straight portions 37 and 39 of conductor 33 lie in respective engagement with the bottoms of grooves 27 and 28. As seen in FIG. 8, conductor 33 is of a diameter slightly shorter than the width of the grooves and markedly smaller than the depth of the grooves. The conductor is fixed to the casing by epoxy cement, as seen in FIG. 2, the cement completely filling the grooves 27 and 28 and completely covering all otherwise exposed portions of the conductor save for the elongated end portion 41, so that the conductor is properly insulated electrically.

The diameter of conductor 33 is significantly smaller than that of aperture 25. The space between end portion 35 and straight portion 39 is such that, considering manufacturing tolerances, end portion 35 will always be disposed within aperture 25 when the conductor is properly oriented and straight portion 39 is in engagement with the bottom of groove 28.

Conductor 34 is in all respects identical to conductor 33 and includes an end portion 43 projecting through aperture 26 and soldered to the remaining end of resistance element 32. Straight portions 44 and 45 of conductor 34 lie in grooves 30 and 31, respectively. Straight end portion 46 projects away at right angles from side 6 and lies in the plane containing end portion 41 of conductor 33 and the longitudinal center line of side 6. Conductor 33 is secured to casing 1, and appropriately insulated, by epoxy cement which fills grooves 30 and 31.

Side 5 of the integral unit constituting half section 3 is a fiat portion of uniform thickness save for the presence of a straight longitudinally extending groove 47 of slightly more than semi-circular transverse cross-section. Groove 47 is provided in the inner surface of side 5 and is of a diameter significantly smaller than that of resistance element 32. The longitudinal edge 48 of side 5 is a flat surface adapted for flush engagement with the inner surface ofthe edge portion 23 of side 6. Groove 47 extends parallel to edge 48 and is spaced therefrom by :a distance sufiiciently greater than the diameter of resistance element 32 that, when the casing is fully assembled, the center line of groove 47 is spaced significantly farther from the outer surface of side 6 than is the center line of resistance element 32.

Side 7 of half section 3 is also a flat portion of uniform thickness and terminates in a fiat longitudinal edge surface 49 adapted for flush engagement with the inner sur face of edge portion 16 of side 4.

Sides 4 and 6 are at right angles, as are sides 5 and 7, and edge surfaces 48 and 49, and the inner surfaces of edge portions 16 and 23, are all so located that, when the two half sections of the casing are properly fitted together, the respective fiat longitudinal edge surfaces 50 and 51 of sides 4 and 6 lie in the same planes as the outer surfaces of sides 7 and 5, respectively. The longitudinal edges at 50 and 51 are chamfered, as indicated respectively at 52 and 53, FIG. 2, and the end edges are similarly chamfered, as indicated at 54 and 55, FIG. 6, so that the two half sections can be secured together by epoxy cement which fills the triangular grooves provided by the chamfers.

As will be understood from a comparison of FIGS. 6, 7 and 10, casing end portion 8 is provided with a slot 56 which extends transversely of the casing and opens both through, surface 10 and through surface 21, the mouth of the slot at surface 10 being closed by portion 4a of side 4. Slot 56 is defined by a flat wall 57, facing side 6, a flat wall 58, facing side 7, and an inner Wall 59 which is of semi-circular transverse cross-section and slants from a point at the longitudinal center line of side 6 to a pivot adjacent the inner surf-ace of side 5 at the end of groove 47. Walls 57 and 58 are parallel to each other and to the transverse edges of casing end portion 8.

Slot 56 and groove 47 accommodate a return conductor 60 which can be of round, gold plated nickel wire of the same diameter as lead conductors 33 and 34. Conductor 60 includes a straight portion 61 disposed in and extend ing for the full length of groove 47, a straight portion 62 extending in engagement with wall 59 of slot 56, and a straight elongated end portion 63 projecting at right angles away from side 6, so that conductor portions 41, 46 and 63 all lie in a common plane at right angles to side 6 and including the longitudinal center line thereof. Slot 56 is completely filled with epoxy cement, so that return conductor 60 is fixed to the casing and only terminal portion 63 is exposed uninsulated. Tip 64 is disposed in groove 47 at face 22 of casing end portion 9.

Casing end portion 8 is provided with a cylindrical recess 65 which opens through face 21 and is coaxially aligned with a through bore provided in end portion 9 and comprising a cylindrical inner portion 66 and a cylindrical outer portion 67, the latter being of larger diameter than portion 66 so that a transverse annular shoulder 68 is provided. A metal lead screw, indicated generally at 69, extends longitudinally of the casing and includes a plain cylindrical tip 70 rotatably engaged in recess 65, a threaded body portion 71 extending between faces 21 and 22, a smaller plain cylindrical portion '72 journaled in bore portion 66, and a larger cylindrical head portion 73 disposed in and projecting outwardly from bore por tion 67, the head portion having a suitable slot 74 to receive an operating tool. At the inner end of portion 72, the lead screw has a transverse annular groove accommodating a U-shaped retaining plate 75 which cooperates with casing end portion 9 to prevent longitudinal removal of the lead screw from the assembled casing, and presents a surface which faces toward the opposite end of the casing and, in effect, is a continuation of face 22. Portions 72 and 73 are joined by a transverse annular shoulder which cooperates with shoulder 68 to limit longitudinal movement of the lead screw inwardly relative to the easing. Adjacent this shoulder, head portion 73 has a transverse annular groove 76 which accommodates an O-ring 77, providing a fluid-tight seal between the lead screw and the casing.

Threaded portion 71 of the lead screw is completely covered by a thin, uniform sheath 78 of electrical insulating material. Advantageously, sheath 78 is in the form of a thin walled tube of synthetic resinous material shrunk onto the threaded body portion of the lead screw in such fashion as to lie in continuous, uniform contact therewith and thus assume precisely the surface configuration of the threaded portion of the lead screw so that the lead screw thus presents a non-conductive threaded surface. Typically, a tube or sleeve of heat shrinkable thermoplastic material having good anti-friction characteristics, e.g., polyethylene or polytetrafluoroethylene, is slipped over the body portion 71 of the lead screw and is then shrunk into tight uniform engagement therewith by application of heat.

Cooperating with the lead screw is a movable contact structure indicated generally at 79 and shown in detail in FIGS. 3-5. Structure 79 is formed as an integral unit from spring sheet metal such, for example, as berylliumcopper alloy, and includes a body portion 80 which is bent cylindrically, and a pair of elongated contact arms 81 and 82 projecting respectively from the longitudinal edge portions 88 and 84 of the body portion. Edge portions 83 and 84 are bent outwardly and the relaxed configuration of the structure is such, as seen in FIG. 3, that the edge portions are in side-by-side engagement with each other, with contact arms 81 and 82 also extending in mutual engagement. Arm 81 terminates in a contact tip 85 bent into shallow U-shaped transverse cross-section, tip 85 being concave relative to arm 82. Arm 82 similarly terminates in a contact tip 86 of shallow U-shaped transverse cross-section, tip 86 being concave relative to arm 81. Arm 81 is slightly longer than arm 82. At a point midway in its length and diametrically across from edge portions 83, 84, the tubular body portion 80 is provided with an inwardly projecting hemispherical dimple 87,

a. FIGS. 3 and 4, of such size as to project into and be closely embraced by the valley of the continuous thread presented by the lead screw, when body portion resiliently embraces the threaded portion of the lead screw.

As will be clear from a comparison of FIGS. 3 and 5, the inner diameter of body portion 80 is significantly smaller than the effective outer diameter of the threaded portion of the lead screw, when the contact structure is in the relaxed condition seen in FIG. 3. The movable contact structure is operatively installed on the lead screw, as the latter is inserted through casing end portion 9, by spreading contact arms 81 and 82 apart and thus increasing the inner diameter of body portion 80 to such an extent that it can he slipped over the tip 70 of the lead screw and onto threaded portion 71, the Contact arms then being released to allow body portion 86 to contract resiliently and forcibly embrace the continuous ridge of the thread presented by sheath '75. Since the effective diameter of the ridge is significantly larger than the normal or relaxed inner diameter of tubular body portion 86, the edge portions 83 and 84 are now spaced apart, as seen in FIG. 5, and contact arms 81 and 82 therefore diverge outwardly to such an extent that contact tips and 86 are spaced apart by a distance substantially greater than the space between resistance element 32 and return conductor 60 in the assembled potentiometer.

Lead screw 69 and contact structure 79 are installed on casing half section 3 before the latter is secured to half section 2. The contact structure, with arms 81 and 82 diverging as seen in FIG. 5, is rotated on the lead screw to bring contact tip 86 into engagement with return conductor 60. Resistance element 32 and its lead conductors having been installed on half section 2, the assembly of half section 3, return conductor 60, lead screw 69 and contact structure 79 is moved laterally (from right to left as viewed in FIG. 2) in such manner that contact tip 85 first comes into engagement with resistance element 32 and edges 48 and 49 then come into proper respective engagement with edge portions 23 and 16, such movement causing the contact structure to be resiliently deformed in such manner that arms 81 and 82 become generally parallel and spaced apart, as seen in FIG. 2. Because of such deformation, and the fact that the contact structure is formed from spring stock, contact tips 85 and 86 are urged resiliently into good sliding engagement with resistance element 32 and conductor 6%), respecti-vely.

Since the combination of arms 81 and 82 is disposed between resistance element 32 and conductor 60, and since body portion 80 firmly embraces sheath 78, the contact structure is held against rotation. When lead screw 79 is turned, engagement of dimple 87 in the valley of the continuous thread presented by sheath 78 causes the contact structure to be moved longitudinally along the lead screw, in a direction depending on the direction of rotation of the screw, with contact tips 85, 86 therefore sliding along resistance element 32 and return conductor 60', respectively.

Faces 21 and 22 of the casing end portions constitute stops to limit the extent of travel allowed the contact structure. When the lead screw has been turned to such an extent that body 80 engages one of faces 21, 22 and turning of the screw in that direction is then continued, body 80 is resiliently deformed in such fashion as to allow dimple 87 to disengage from the valley of the screw thread once during each turn of the screw, so that the excessive turning of the screw is permitted without damage. Such deformation is essentially an increase in diameter of body portion 80, with edge portions 83 and 84 spreading further apart and therefore moving slightly around the periphery of sheath 78 in directions away from side 6. In this connection, the lengths of arms 81 and 82, and particularly of contact tips 85 and 36, are adequate to assure that the contact tips will remain properly engaged with the resistance element and return conductor.

Though one particularly advantageous embodiment has been disclosed to illustrate the invention, it will be clear to those skilled in the art that various changes and modifications can be made therein Without departing from the scope of the invention as defined in the appended claims.

Attention is called to my copending application Ser. No. 581,943, filed concurrently herewith, which is directed to the lead screw and movable contact structure hereinbefore described.

What is claimed is:

1. In a potentiometer, the combination of an elongated hollow casing having two spaced opposite sides and a third side extending between said two sides; an elongated resistance element mounted within said casing and extending parallel to one of said two sides, said one side having an aperture directed toward the location of one end portion of said resistance element, and a first exterior groove communicating with said aperture and extending to the junction of the outer surfaces of said one side and said third side, said third side having a second exterior groove communicating with said first groove and extending from said junction to a point within the confines of said third side; lead means including a first conductor having a first end portion projecting through said aperture and connected electrically and mechanically to said one end portion of said resistance element, a portion extending through said first groove to said junction, a portion extending from said junction through said second groove, and a second end portion projecting away from said third side; said lead means also comprising a second conductor electrically connected to the other end portion of said resistance element; a lead screw carried by and extending longitudinally within said casing; and movable contact means slidably engaging said resistance element and operatively coupled to said lead screw, rotation of said lead screw driving said contact means along said resistance element;

2. A potentiometer according to claim 1, wherein said one side and said third side of said casing are parts of an integral unit and extend at right angles to each other, said resistance element extends along the junction between the inner surfaces of said one side and said third side, and said aperture is spaced from said third side by a distance such that said one end portion of said resistance element is disposed generally between said aperture and said third side.

3. A potentiometer according to claim 1, wherein said one side has a second aperture directed toward said other end portion of said resistance element, and a third exterior groove communicating with said second aperture and extending to the junction of the outer surfaces of said one side and said third side, said third side has a fourth exterior groove communicating with said third groove and extending from said junction to a point within the confines of said third side, said second conductor having a first end portion projecting through said second aperture and connected electrically and mechanically to said other position of said resistance element, a portion extending through said third groove to said junction, a portion extending from said junction through said fourth groove, and a second end portion projecting away from said third side.

4. A potentiometer according to claim 2, wherein said casing includes a fourth side spaced from and opposite to said third side, said fourth side and the other one of said first-mentioned spaced opposite sides being parts of an integral unit and extending at right angles to each other, said last-mentioned integral unit including two longitudinally spaced end portions respectively closing the ends of said casing, said end portions being of rectangular transverse cross-section, the length of said third side bieng substantially equal to the space between the adjacent faces of said end portions and said third side being disposed between said end portions.

5. A potentiometer according to claim 4, wherein said one of said first-mentioned spaced opposite sides has end portions overlapping the corresponding surfaces of said end portions of said last-mentioned integral unit.

6. A potentiometer according to claim 4, wherein the outer surface of said one side is fiat; said one side has a longitudinally extending inner shoulder and is relatively thicker in the area between said shoulder and said third side, said aperture and said first groove being located in the relatively thicker portion of said one side, said one side being relatively thinner in the area between said shoulder and the edge thereof opposite said third side; and said lead screw is disposed between said thinner portion of said one side and the corresponding portion of the other of said first-mentioned two sides.

7. A potentiometer according to claim 1, wherein said one side and said third side of said casing are parts-of a first integral unit and extend at right angles to each other, said resistance element extending along the junction between the inner surfaces of said one side and said third side; said casing including a fourth side spaced from said third side, said fourth side and the other one of said first-mentioned spaced opposite sides constituting parts of a second integral unit and extending at right angles to each other, said second integral unit including two longitudinally spaced end portions respectively closing the ends of said casing; said lead means including a third conductor carried by said second integral unit and extending along the inner surface of said other one of said first-mentioned spaced opposite sides parallel to and spaced from said resistance element; said lead screw extending parallel to and adjacent to said fourth side; said movable contact means comprising body means operatively coupled to said lead screw, and two contact arms projecting from said body means and each bearing a contact, one of said contacts slidably engaging said resistance element, the other of said contacts slidably engaging said third conductor.

8. A potentiometer according to claim 7, wherein said contact arms are electrically interconnected; one of said end portions including a passage having one end opening into the interior of said casing adjacent said inner surface of said other one of said first-mentioned spaced opposite sides, the other end of said passage opening through said third side, said third conductor of said lead means extending through said passage and having an end portion projecting away from said third side.

9. A potentiometer according to claim 8, wherein the inner surface of said other one of said first-mentioned spaced opposite sides comprises a straight groove extending between said end portions; said third conductor of said lead means being disposed in said groove and terminating short of the other of said end portions.

10. A potentiometer according to claim 8, wherein said third conductor of said lead means extends along said other one of said first-mentioned spaced opposite sides in a straight line spaced from the inner surface of said third side, said passage slanting toward the longitudinal center line of the outer surface of said third side.

11. In a potentiometer, the combination of a generally rectangular casing comprising a first integral unit including a first fiat side, a second integral unit including a second fiat side and two longitudinally spaced end portions, said integral units being secured together with said first and second flat sides parallel and spaced apart; an elongated resistance element mounted in said casing and extending longitudinally thereof adjacent said first flat side; a return conductor including a main portion disposed within said casing and extending longitudinally thereof along said second fiat side, one of said end portions having a transverse slot opening away from said second side and slanting from a location adjacent one end of said main portion of said return conductor to- 9 ward and opening through a third side of said casing; said slot having an inner edge slanting from said second side to a point at the longitudinal center line of said third side, said return conductor including a portion extending along said inner edge of said slot and another portion 5 element and said main portion of said return conductor; and lead means connected to said resistance element.

References Cited UNITED STATES PATENTS 3,117,298 1/1964 Grunwald 338-183 X 3,239,790 3/ 1966 Layland 338---183 3,271,721 9/1966 Gorden 338-183 10 ROBERT K. SCHAEFER, Primary Examiner.

H. J. HOHAUSER, Assistant Examiner. 

1. IN A POTENTIOMETER, THE COMBINATION OF AN ELONGATED HOLLOW CASING HAVING TWO SPACED OPPOSITE SIDES AND A THIRD SIDE EXTENDING BETWEEN SAID TWO SIDES; AN ELONGATED RESISTANCE ELEMENT MOUNTED WITHIN SAID CASING AND EXTENDING PARALLEL TO ONE OF SAID TWO SIDES, SAID ONE SIDE HAVING AN APERTURE DIRECTED TOWARD THE LOCATION OF ONE END PORTION OF SAID RESISTANCE ELEMENT, AND A FIRST EXTERIOR GROOVE COMMUNICATING WITH SAID APERTURE AND EXTENDING TO THE JUNCTION OF THE OUTER SURFACES OF SAID ONE SIDE AND SAID THIRD SIDE, SAID THIRD SIDE HAVING A SECOND EXTERIOR GROOVE COMMUNICATING WITH SAID FIRST GROOVE AND EXTENDING FROM SAID JUNCTION TO A POINT WITHIN THE CONFINES OF SAID THIRD SIDE; LEAD MEANS INCLUDING A FIRST CONDUCTOR HAVING A FIRST END PORTION PROJECTING THROUGH SAID APERTURE AND CONNECTED ELECTRICALLY AND MECHANICALLY TO SAID ONE END PORTION OF SAID RESISTANCE ELEMENT, A PORTION EXTENDING THROUGH SAID FIRST GROOVE TO SAID JUNCTION, A PORTION EXTENDING FROM SAID JUNCTION THROUGH SAID SECOND GROOVE, AND A SECOND END PORTION PROJECTING AWAY FROM SAID THIRD SIDE; SAID LEAD MEANS ALSO COMPRISING A SECOND CONDUCTOR ELECTRICALLY CONNECTED TO THE OTHER END PORTION OF SAID RESISTANCE ELEMENT; A LEAD SCREW CARRIED BY AND EXTENDING LONGITUDINALLY WITHIN SAID CASING; AND MOVABLE CONTACT MEANS SLIDABLY ENGAGING SAID RESISTANCE ELEMENT AND OPERATIVELY COUPLED TO SAID LEAD SCREW, ROTATION OF SAID LEAD SCREW DRIVING SAID CONTACT MEANS ALONG SAID RESISTANCE ELEMENT. 